What are the characteristics of normal alpha rhythm on EEG?

Updated: Oct 09, 2019
  • Author: Selim R Benbadis, MD; Chief Editor: Helmi L Lutsep, MD  more...
  • Print
Answer

Answer

The normal alpha rhythm has the following characteristics:

  • Frequency of 8-12 Hz - Lower limit of normal generally accepted in adults and children older than 8 years is 8 Hz

  • Location - Posterior dominant; occasionally, the maximum may be a little more anterior, and it may be more widespread

  • Morphology - Rhythmic, regular, and waxing and waning

  • Amplitude - Generally 20-100 mV

  • Reactivity - Best seen with eyes closed; attenuates with eye opening

    A 10-second segment showing a well-formed and well A 10-second segment showing a well-formed and well-regulated alpha rhythm at 9 Hz. Note that it is very regular, rhythmic, waxing and waning, and posterior dominant. The contrast between the first and second halves of the page illustrates the reactivity of a normal alpha rhythm, with attenuation upon eye opening.
    Fleeting alpha. At times, as shown here, the alpha Fleeting alpha. At times, as shown here, the alpha rhythm can be identified only in very brief bursts and often immediately after eye closure. If normal in frequency, this is normal.
    This is an example of an alpha rhythm with a wider This is an example of an alpha rhythm with a wider distribution than is typical. If frequency and reactivity are normal, this is another variation of normal. A similar EEG pattern can be seen in patients in a coma (ie, alpha coma), but in these situations it is usually unreactive.
    This is an example of "slow alpha variant." The pa This is an example of "slow alpha variant." The patient's alpha rhythm at 12 Hz is seen in the second half of the sample. The first half shows a subharmonic at half that frequency, and this is the "slow alpha variant."
    An example of a typical normal alpha rhythm, showi An example of a typical normal alpha rhythm, showing clear attenuation upon eye opening (second half of page).
    Alpha rhythm with somewhat "spiky" or sharply cont Alpha rhythm with somewhat "spiky" or sharply contoured morphology. When fragmented (eg, in drowsiness), this can be misinterpreted as sharp waves.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!